(1) Field of the Invention
The present invention relates to a process for forming 2-alkylbenzimidazole layer (hereinafter referred to as "chemically formed layer") having an excellent heat resistance on the surface of copper or a copper alloy. More particularly, the present invention provides a process suitably used for the preflux treatment of copper on printed circuit board or a flexible printed circuit board.
(2) Description of the Related Art
Processes for chemically forming a layer of an imidazole compound having a long-chain alkyl group at the 2-position on the surface of copper or a copper alloy are disclosed, for example, in Japanese Examined Patent Publication No. 46-17046, No. 48-11454, No. 48-25621, No. 49-1983, No. 49-26183, No. 58-22545 and No. 61-41988, and Japanese Unexamined Patent Publication No. 61-90492.
Furthermore, as the method for forming a chemically formed layer of a benzimidazole compound, Japanese Unexamined Patent Publication No. 58-501281 discloses a method using 5-methylbenzimidazole.
Recently, as the method of bonding electronic components to a printed circuit board, there has been frequently adopted a surface mounting technique and surface mount technology (SMT) was taken in order to obtain high density packaging of electric components on the printed circuit board. In this technology, the printed circuit board is exposed to high temperature due to reflow soldering, temporary fixing of chip components, and double-sided surface mounting of electric components.
However, the conventional chemically formed layer of an imidazole having a long-chain alkyl group at the 2-position, which is selectively formed on copper, is degraded at a high temperature though the film is stable at a temperature close to room temperature, and as a result sometimes, soldering becomes impossible.
In the known method for the surface treatment of copper or copper alloy with a benzimidazole compound, since the benzimidazole compound is easily soluble in water, a chemically formed layer having a desirable thickness, that is, a thickness of at least 0.08 .mu.m, cannot be formed, and the obtained film can hardly exert the function of protecting copper from oxidation and corrosion under heating.
Accordingly, formation of a preflux coating film having a highly improved heat resistance on the surface of copper or copper alloy, which can cope with the method of surface mount technology of a printed circuit board, is eagerly desired.